It is desirable to insulate the combustion chamber in an internal combustion engine to reduce heat loss, improve engine efficiency, improve emission quality, and maximize specific power output. One method to achieve this purpose is to apply an insulating ceramic coating to the combustion chamber defining components. A wide range of ceramics have favorable thermal barrier and thermal expansion characteristics, and may be easily applied by a variety of coating processes and modified to meet a wide range of functional requirements.
One known ceramic coating is a very thin layer from 0.0002 to 0.001 inch thick as described in U.S. Pat. No. 4,074,671. This patent teaches that even an extremely thin ceramic coating can increase combustion chamber temperature. However, coatings of such a thickness do not cause an increase in temperature sufficient to significantly enhance engine performance.
Much more popular are thicker ceramic coatings on the order of 0.020 to 0.110 inch, such as described in U.S. Pat. Nos. 4,419,971 and 4,495,907. It has been well recognized that these thicker coatings increase the cycle mean temperature of the combustion chamber. However, numerous and significant problems are caused by such thick coatings. Most importantly, thick coatings are unsuitable for gasoline engines because they raise the temperature of the fuel-air mixture to such a high level as to cause preignition, knocking, and breakdown of lubricants. Moreover, the volumetric efficiency of the engine is reduced due to the increased air or air-fuel temperature caused by heat transfer from the combustion chamber during the intake cycle. Finally, thick coatings tend to chip, crack and separate from their metal substrate due to the thermal expansion characteristics of the metal substrate, and low reliability associated with these coatings.